Unopened DCB Passed to Checkpoint

Sequential data sets
- When a sequential data set's disposition is NEW or OLD, the checkpoint entry is written at the beginning of the data set, even if a checkpoint entry was written on a previous call.
- When a sequential data set's disposition is MOD, the checkpoint entry is written after the last entry existing in the data set.
PDSs
- When the data set is partitioned, each checkpoint entry is a member, and its checkid is its member name. After it writes a checkpoint entry, the checkpoint routine executes the STOW macro to add the checkid of the entry to the directory of the data set.
- If the data set is partitioned, regardless of its disposition, the checkpoint entry is written after the last entry existing in the data set.
- If an identical checkid already exists in the directory, the related address of a member is changed and becomes the address of the new checkpoint entry. The initial disposition specified for the checkpoint data set has no effect on the STOW operation.